Don't kill the base register when expanding strd.

When an strd instruction doesn't get the registers it wants, it can be
expanded into two str instructions. Make sure the first str doesn't kill
the base register in the case where the base and data registers are
identical:

  t2STRi12 %R0<kill>, %R0, 4, pred:14, pred:%noreg
  t2STRi12 %R2<kill>, %R0, 8, pred:14, pred:%noreg

<rdar://problem/11101911>

llvm-svn: 153611